Here's a few I've liked doing these past few weeks. I'm gonna make up names on a few of these, haha.
Bodybuilding.com has some great info.
dumbbell curl lying against an incline -
The example shows the dude using a barbell...just use dumbbells in your case
Dumbbell Alternating Curls (Burnout) -
These are regular dumbbell curls, but the burnout part adds some definite fatigue to the routine. Start with the heaviest set of weights you can use to curl, while maintaining proper form. Do as many reps as you can. Drop down to the next lightest weight (usually a 5 pound drop) and do as many reps with those that you can. Continue stepping down as far as you'd like. I actually go down to 10 lb weights. I'll usually do these once a month as the last exercise whenever I work biceps.
Dumbbell Curls with Resistance -
Grab a pair of dumbbells and let's say you're gonna start curling with your right arm. Before you start working your right arm, lift your left arm so that your forearm is parallel to the ground (palm facing up) and hold it there. Do a normal set of bicep curls using your right arm...let's say, 10 reps. After your final rep, bring up your right arm so that it's parallel to the ground (palm facing up), then begin doing curls with your left arm. Again, 10 reps. That's one set. Play around with resting between sets or doing three sets back to back without rest.
Negatives are really useful for arms and other body parts too.