I've done a few sprint triathlons (1/4 mile swim, 15 mile bike, 3 mile run), but I can't imagine I'll ever go farther (I'm a horrible swimmer).
The Lake Mills, Wisconsin triathlon is a good one. It's social but still competitive. The Manitowoc, WI triathlon is a great one for beginners. I've signed up four times for the Door County Triathlon, but I was really lame and I blew off the first two because the weather was perfect for sailing.
Update 8/9/08: Ok, I'm a bad man. I took some time off and actually did a sprint triathlon on my own. My time was 1:33:14. Now, four days after my trial run, I've decided I'm not doing the Oshkosh triathlon. The purpose for entering these events is to get in shape for them. That's it. And since Oshkosh starts at 7 I would have to leave Green Bay by 5 AM. That's not that big a deal, but I'm doing the triathlon by myself and I won't know anyone there. So why go?
Update 7/15/09: I did the Door County Triathlon! Had a blast and finished in the middle of my age group. I rode my mountain bike and did the 18-mile ride in just under an hour. No one passed me on the 3-mile run. Now I just have to get better at swimming...
Update 7/20/10: Another Door County Triathlon! I biked faster on my mountain bike. At the end of the bike another participant said, "If you had my bike you'd be kicking my ass!" My run was not so good...