Fire Ants - very common, you will most certainly encounter these. pretty painful bites that stick with you for a while
Scorpions are very rare here, but can still be found.
Velvet ants also rarely seen
Black Widows - you will see these, I see them often
Brown Recluse - you won't typically know what you're looking at but also a common occurrence
Copperheads - common if you live near a wooded area
http://scienceblogs.com/clock/upload/2007/02/copperhead.jpg (broken link)
Water Moccasins - less common but if you live near water you may see one
rattle snakes - I've never seen one and have heard very few stories of sightings in our area
Don't confuse the Brown Recluse with the non-poisonous Wolf Spider which is a VERY common spider here