For casual use you have many choices (like the ones other's posted). Anything above the bargain-basement price level will probably work for you.
But I will suggest you find a 'bundle' on sale with two LION batteries. Two big advantages:
1) If you do find yourself doing a bigger job, you can swap batteries and keep going, the other will probably be charged by the time you run down the other.
2) Batteries degrade over time. My unit is now ~ 8 years old, the batteries (older style NiCads) don't last so long, but since I can just swap, I get by just fine. If I ever foresee a big job coming up, I'll buy a new one, but having the dual batteries has really extended the useful life of the purchase for me.
Also, a bundle with a second battery often include a flash light. It's really nice to have a flashlight powered by a powerful, rechargeable battery.
I got mine on sale at Costco (around $40-$60?), but that was 2006, similar to this bundle:
Amazon.com: Kawasaki 840110 Black 19.2-Volt Drill Kit: Home Improvement
These are NiCad, not as good as LION batteries, but cheaper, and likely good enough for casual use.
-ERD50