Blue Moon Render Tools was a shareware ray tracer (+radiosity) developed my Larry Gritz. It provides a very full implementation of the Renderman standard. Unfotunatly it has been dicontinued, along with its commercial sucessor Entropy for legal reasons.
Advanced Rendering Technologies might be the best of both worlds. They've developed a custom processor that is esspecially good at ray tracing. This allows complex lighting effects at PRMan speeds.
Render Dot C, is a scanline renderer running on NT, SGI, HP, and Linux, which is broadly comparable to PRMan. A resolution limited "broadcast" version is also available at a very competetivly price.
Aqsis is an open source scanline renderer. Though still relativly new, it seems to be a pretty good foundation. Its free to download and as it's open source - if you don't like it then make it better.
Air is a hybird scanline/ray tracing renderer for windows, and Linux, available from Sitex Graphics.
Angel is my own renderer available for free download.
3Delight, is a hybrid renderer, which is currenltly availabel for free download.
RenderMan renderer projects seem to be popular to start, and then the once keen developer discovers its a rather harder slog than they figured. A complete list of renderers, past, present and future can be found on the Render Dot C site.
Pixar support Maya through there MTOR plugin, part of the Renderman Artist Tools (RAT) package. Textures are attatched using glimpse rather than the Maya multilister which can make transitioning a little tedious. RAT includes the Alfred network rendering system which allows excellent control over large and complex networks of machines which can be used for rendering when not in use.
To access Renderman from SoftImage requires SoftMan by Animal Logic. UNforunatly it doesn't currently work with XSI.
Animal Logic also have a plugin for 3D Studio Max, (called MaxMan) and Maya.
Well I'd naturally recommend my own book: Essential RenderMan fast. Once you've read that you might also like to look at some of the following.
The RenderMan bible is "The RenderMan Companion" by Steve Upstill (Addison Wesley). It explains the C API to generate RIB files, and the "SL" shading language. It serves as both a tutorial and a reference. Though it probably falls slightly short in both roles its essential for anyone serious about RenderMan.
Unfortunatly the Companion is a little dated, and misses out on a few recent additions to the language. Details of these can be found in the Renderman Release notes on the Pixar web site.
Modeling, A Procedural Approach" by David Ebert
Chapter two of "Texturing & Modeling" (written by Darwyn Peachey) is perhaps the most usefull chapter of the book. Similar information can be found in the excellent RenderMan Notes and related web pages by Stephen F. May. Also check out the RenderMan Repository
Finally "Advanced Renderman : Creating Cgi for Motion Pictures" by Anthony A. Apodaca and Larry Gritz. Though the other books cover things in much more detail, this one provides a good up to date summary of their contents, plus lots of new goodies.
The final renderman resource is the news group comp.graphics.rendering.renderman. This is populated by a mix of inhabitants from the totally naive to a number of Pixar employees. See Ya There...