It's widely known that the OS X Services infrastructure was completely overhauled (and made significantly more useful) in Snow Leopard. What virtually no one seems to be aware of is that Apple released a 'Services Manager' tool as long ago as 2009, which offers significant advantages over using the built-in support in the Keyboard preference panel.
In Snow Leopard (and now in Lion), it's possible to create your own Services (using e.g. Automator), and it's also possible to disable or enable Services provided either by applications or the system itself, by going to the Keyboard Shortcuts tab of the Keyboard preference panel.
However, the Keyboard preference panel has a number of limitations when it comes to editing Services, and unfortunately these limitations add up to it being just about unusable in practice. For example, while you can activate or deactivate a particular Service, there's no way to specify whether that Service will only be available on the Services menu, or (much more usefully) on relevant contextual menus as well. OS X seems to decide which Services appear on contextual menus based on its own arcane rules.
Services are listed by name only in categories based on their function. There's no easy way to see whether a particular Service is offered by the system itself, a third party application, or (say) an Automator workflow in /Library/Services or ~/Library/Services.
Back in pre-10.6 days, when there was no officially supported way of editing Services, a free tool called Service Scrubber by Many Tricks Software could be used for this purpose. Due to the changes made to Services in Snow Leopard, Service Scrubber no longer works in 10.6+. I've often wished there were a modern replacement.
As it turns out, there is! A tool named Service Manager is provided by Apple itself, hidden away on its macosxautomation.com website. You can download it here, and you'll find it provides all the functionality mentioned above, and more. Services Manager was intended for use with Snow Leopard, but I have so far found no problems using it in Lion.
[crarko adds: Works the same in either Lion or Snow Leopard. This is a great little utility.
Note: since some people seem to be freaking out in the comments here is some referential information about Sal. He's pretty much the boss of Services.]
Mac OS X Hints