I'm building a deck. Joists are sitting on top of the beam and cantilever over the end of the beam a bit.
What is the appropriate way to connect the rim (end of the deck that is perpendicular to the joists) to the joists? I've seen people toenail it to the joists, but my understanding is you shouldn't nail into the end grain of the joists. Is there a connector built for this purpose (I'm thinking there is probably a 90 degree plate)?
As Tester101 mentioned, in this application there is close to no load on the rim joist, since your actual joists are supported by the beam. Nailing through the rim joist into the end of the other joists will be fine. I've built plenty of decks like this.
Here's a study that was done on end nailing, and the relevant recommendations:
Recommendations
Nails driven into the end grain of wood do resist both static and impact loads. However, additional research is required in several areas. Of the 4,723 data points on the relationship of specific gravity to nail withdrawal strength, only 294 data points were from tests of nails with a diameter larger than 2.52 mm (0.099 in.). Therefore, research is needed on endgrain withdrawal capacity of nails with a diameter greater than 4.1 mm (0.163 mm) so that Equation (5) can be further developed.
Furthermore, none of the studies evaluated repetitive loading of nails from the end grain since joints in service typically do not experience strictly static or impact loading. Repetitive loading may cause end-grain withdrawal capacity to significantly decrease over time.
There is conflicting data on how threaded nails perform in comparison with smooth shank nails when withdrawn from the end grain. Three studies showed an increase in nail withdrawal capacity whereas one study showed a decrease. Since the use of threaded fasteners in construction is increasing, more research is needed to determine the effects of surface characteristics on end-grain withdrawal.
Finally, more research is needed to determine the long-term withdrawal strength of nails driven into end and side grain. The data in this report suggest that over time end-grain and side-grain withdrawal capacity may become equivalent. Very few test replicates measuring the effect of time delay on withdrawal strength were conducted, and more tests need to be run to clarify these limited observations.
If you still feel like you need more support, yes, you can find something like this: 
at most hardware stores. If you're working with pressure treated lumber remember you will need galvanized plates and connectors.
It sounds like you need joist hangers
The advantage of these is the nails into the joists run across the grain, not in parallel to it (as they would if you end nailed them).
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