W3C (World Wide Web Consortium): As the standards body for the World Wide Web, the W3C is the primary mover and shaker behind the Synchronized Multimedia Integration Language. Start here if you want the official word on SMIL:
W3C Synchronized Multimedia Homepage (http://www.w3.org/AudioVideo/)

General SMIL Sites: If you are looking for more general information about SMIL, there are a number of sites available.
Just SMIL (http://JustSMIL.com) - A great site in its day but sadly dwindling since my departure. Still holds a good number of helpful articles.
The CWI SMIL Page (http://www.cwi.nl/~media/SMIL/) - CWI (Centrum voor Wiskunde en Informatica) is the National Research Institute for Mathematics and Computer Science in the Netherlands.
Universal SMIL (http://www.empirenet.com/~joseram/universal/universal.html) - A site logging efforts toward SMIL that can play anywhere.
SMIL Defined at Webopedia (http://webopedia.internet.com/TERM/S/SMIL.html) - A definition of SMIL and a small set of starting links.
WebReview on SMIL (http://www.webreview.com/1999/10_08/designers/10_08_99_1.shtml) - A look at new directions in SMIL.

SMIL 2.0: If you want to live on the cutting edge of SMIL, SMIL 2.0 is the choice for you. Adoption is mostly limited to RealOne and Internet Explorer but the extra capabilities are worth the extra effort:
SMIL 2.0 Specification (http://www.w3.org/TR/smil20/)
HTML+TIME in Internet Explorer (http://msdn.microsoft.com/library/default.asp?url=/workshop/author/behaviors/time.asp) - Microsoft's documentation on the HTML+TIME flavor of SMIL.
SMIL 2.0 in RealOne (http://service.real.com/help/library/guides/productionguidepreview/HTML/realpgd.htm) - RealNetworks's documentation on SMIL 2.0 in their newest RealPlayer.

SMIL 1.0: SMIL 1.0 is friendly and easy. You will not find the extra power of SMIL 2.0 but you will find more tools and an established track record of support. It is well supported by RealPlayer and QuickTime:
SMIL 1.0 Specification (http://www.w3.org/TR/REC-smil/)
WebTechniques: Synchronized Multimedia on the Web (http://www.webtechniques.com/archives/1998/09/bouthillier/) - A primer on SMIL 1.0.
WebMonkey: Multimedia for the Masses (http://hotwired.lycos.com/webmonkey/00/41/index4a.html?tw=authoring) - An overview of SMIL 1.0.
WebReview: Making Sites SMIL (http://www.webreview.com/2000/05_05/designers/05_05_00_1.shtml) - Building a basic SMIL presentation.
RealNetworks Assembling a Presentation in SMIL (http://service.real.com/help/library/guides/production8/htmfiles/smil.htm#100023) - From the RealSystem Production Guide for RealPlayer.
Apple QuickTime SMIL (http://www.apple.com/quicktime/authoring/qtsmil.html) - SMIL 1.0 in QuickTime. Also see (http://www.apple.com/quicktime/authoring/qtsmil2.html).
Helio SMIL Tutorial (http://www.helio.org/products/smil/tutorial/index.html) - A tutorial on SMIL by the folks who offer a Java SMIL player.