There are a bunch of off camera trigger options like PocketWizard. It's the gold standard but also the most pricey.
On the cheap end, you've got Ebay triggers and the Cactus triggers. Less range, reliability is a wild card For casual use where the flash failing to fire part of the time is okay, they may be a good option.
The next price tier up is Cybersyncs and Elinchrom triggers. Improved reliability, both are also associated with there own like of studio stoves to make better use of them. I've used the Cybersyncs and had good luck with them. A friend went with the Elinchom triggers over the PocketWizards and is happy. In 'normal' conditions any of theses would probably be fine.
A newer entry is RadioPopper. Their big thing is supporting Ttl (automatic flash) though I know very little about them as their product came out after I made my trigger purchase.
Lots of variables... reliability, number of channels, grouping lights, range, automatic support, integration with studio lights, profile on the camera, battery type used, etc.