http://www.kenhamady.com/bookmarks.html has a list of viewers/schedulers etc you may want to checkout
I definitely agree it's advisable to go with a 3rd party applications (from a cost + functionality perspective). Also, avoid letting them edit the report as Crystal Reports can be more complex than meets the eye (end users are likely to break them!).
Unfortunately i have no first hand experience with viewers, i've always worked with applications which have a crystal viewer built in OR written my own.
Good luck!
L