Eye arrangements for Zora & Anahita can look similar. I'd say look for the stripes on the legs of Zora and the more broken pattern on Anahita.
Zora
Anahita
"
Cheiracanthium are distinguished from the closely related species of
Clubiona by lacking a conspicuous dorsal groove in the midline of the
carapace, the lack of a cluster of long curved erect
setae at the anterior end of the abdomen, by having the first pair of legs the longest, and by having a long, pointed spur extending back from the
cymbium over the
tibia. Species of
Cheiracanthium are also characterized by having relatively thinner legs and more robust bodies than
Clubiona." From article by Jørgen Lissner seen
here.
Hibana have the dorsal groove, but also have brown markings on the carapace. They also almost always have two brown rows of spots running down the length of the abdomen.
Clubionidae 
Cheiracanthium 
Hibana
The following are generally similar looking, but are easily told apart by looking at the length of the
pedipalps and the
eye arrangements. Some people might also confuse some
cellar spiders with these because they have the "violin" mark, but I don't think they look very similar. There is one more spider that has just been found in Florida, Cithaeron praedonius. You can see images
here.
Loxosceles 
Kukulcania (male) 
Titiotus
While each of these three has its own distinct look their general appearance and coloring may confuse some people. Especially some Tibellus and Pisaurina can look very similar.
Tibellus 
Pisaurina 
Larinia 
Arachosia cubana 
Thanatus
Coloring varies on these, especially in different lighting. The best way to tell these apart is to look at the
eye arrangement and the
chelicerae. Dysdera have only six eyes and very large chelicerae compared to Trachelas which has eight eyes and more normal sized chelicerae.
Trachelas 
Dysdera
These are easy to tell apart if you get a good shot of the
eye arrangement. Modisimus have eight eyes and Scytodes have six.
Modisimus 
Scytodes
Mimetus & A. miniatus have similar markings, but
Mimetus has longer legs. These might also look similar to
Modisimus & Scytodes above. However
Modisimus has much longer, thinner legs &
Scytodes has thinner legs and a different eye arrangement.
Metellina mimetoides can also look very similar to
Mimetus. Look for the two white comma shaped marks on the anterior of the abdominal hump on
Metellina mimetoides, and look for the longer tibiae and curved femurs on
Mimetus.
Mimetus 
Araneus miniatus 
Metellina mimetoides
Again
eye arrangement is key when pattern is not obvious.
Dolomedes tenebrosus 
Pardosa
The dorsal abdominal pattern of the following might be confusing, but
D. triton can be separated from the
Pirata/Piratula by the markings on the carapace. When seen in the field (not just an image) it's obvious that the
Dolomedes are much larger spiders.
Dolomedes triton 
Pirata or Piratula
Amaurobiidae, Amphinectidae, Cybaeidae, Dictynidae, Tengellidae, Zorocratidae
Both Cybaeidae and Amaurobiidae have eyes in two transverse rows with similar carapace shapes. SONA says that Cybaeidae differs from other families by spinneret characters (3 longitudinally arranged pairs), ventral tibial macrosetae (numerous and conspicuous) and smaller size (1-14 mm). In my opinion Cybaeidae may appear to have longer legs in relation to its body than Amaurobiidae.
M. simoni has three pairs of ventral macrosetae on
tibia and
metatarsus;
calamistrum in single row. Amaurobiidae does not have ventral macrosetae, so shots showing the ventral hairs (side views) will be valuable. I think I see the hairs on tibias III & IV of Lenny Vincent's image seen
here.
While the color and markings of some
Cybaeopsis & Callobius spiders look nearly identical the shape of the carapace is drastically different. Look for the wide front & eyes in the genus
Callobius.
Metaltella 
Titanoeca 
Callobius 
Cybaeus
Finally you can separate
Cybaeus from
Cryphoeca exlineae by looking for the white hairs on the abdomen of the latter and Grammonata can be separated by its less striped legs. Grammonata can also be separated from Meriola by its thinner legs.
Cryphoeca exlineae
Grammonata
Meriola
The following look very similar, but the Plectreurys have thickened front
tibia or lateral
apophyses on the front tibia.
Plectreurys 
Kukulcania (female)
We aren't sure how to separate Phrurotimpus from the Liocranidae or whether they even look that similar. We may have some placed incorrectly in the guide. Some comments
here.
Phrurotimpus 
Liocranidae
At this point we aren't sure how to separate
Anyphaena from the
Agroeca or whether they even look that similar. We may have some placed incorrectly in the guide. More research is being done. After reviewing the photos online my initial guess is that we're confusing live specimen patterns of
Anyphaena with dead specimen patterns of
Agroeca. I don't think that the patterns of
Agroeca are the same when the spider is living. Adding to the confusion is the possibility of juvenile specimen patterns looking completely different than the adults patterns.
Anyphaena
Agroeca
Unsure
Dirksia cinctipes
Anyphaena pacifica
Both spiders have a 'Steatoda look' and white mottling on the abdomen. T. gemmosum may have a different pattern of white. More images needed.
T. gemmosum 
S. triangulosa
Between Enoplognatha & Zygiella (& Parazygiella) look for the heavier black line in the center of the abdomen of Enoplognatha. Also Enoplognatha pattern ends with a horizontal black line on the posterior of the abdomen.
Enoplognatha marmorata 
Zygiella
Both groups of spiders can have green legs & green/tan carapaces with a stripe down the middle. Looking closely at the dorsal pattern can differentiate between these. Also, the hairs on Mangora's legs are thicker and longer and it is missing the dark line around the carapace that is found in Leucauge. Neriene can also be confused with these two.. see below.
Leucauge 
Mangora
Mangora 
Neriene 
Leucauge
Not sure on field marks yet. May be able to tell by spinnerets & eyes.
Castianeira 
Castianeira 
Micaria 
Sergiolus
Hypselistes vs. Hypsosinga
Best clue is to record the spider's web type. Kyron has some new tips:
I think shape of the carapace may be enough to tell Hypselistes from Hypsosinga. Hypselistes seems to have a fairly typical erigonine face with AMEs closer together than PMEs, and a tall clypeus. The front looks pretty round when viewed from above, which I've also seen a lot in erigonines.
In contrast, my impression of Hypsosinga is that the AMEs and PMEs form a rectangular shape, the clypeus is kind of short, and the carapace is more cornered off at the front.
Finally I notice some difference in the chelicerae. Hypsosinga's just "look" Araneid in nature to me. I can't figure out quite how to describe it though.
Hypsosinga 
Hypselistes
Males of the following genera may be confused because of their constricted abdomens. However,
Microlinyphia males have a long wiry embolus that turns a full circle larger than the pedipalp itself; both
Neriene & Microlinyphia males have a much longer carapace than
Coleosoma.
Coleosoma 
Microlinyphia 
Neriene
Cyclosa turbinata 
Asagena
**There are also some spiders that look more like other insects. Here is an example of a spider that looks like something in another order.
Obviously if you looks closely you'll see one is a spider, one is a broad-headed bug and one is an ant. However, they have similar coloring and shapes.
Synagelinae 
Alydidae 
P. californicus
