Client-Side Editors: There are a variety of client-side editors available for most platforms. This is one area where the SMIL community continues to (and needs to) grow by leaps and bounds:
Adobe GoLive 6 (http://www.adobe.com/products/golive/main.html) - The latest release of this Mac and PC web editor supports SMIL 1.0 and SMIL Basic for RealPlayer, RealONE, QuickTime, and Nokia MMS.
Confluent Technologies Fluition (http://www.confluenttechnologies.com/fluition.html) - Mac and PC editor for streaming multimedia presentations.
ORATRiX GRiNS (http://www.oratrix.com/GRiNS/index.html) - A SMIL 1 and 2 editor for RealOne.
RealNetworks SMILGen (http://www.smilgen.org/) - A Windows and Unix/Linux authoring tool for SMIL 1, SMIL 2, and RealPix.
RealNetworks RealSlideshow (http://www.realnetworks.com/resources/slideshow/index.html) - A RealPix based Windows tool that is still the nicest example of a "SMIL" authoring tool made for creative people.
Macromedia Flash and HomeSite (http://www.macromedia.com/software/) Basic support for SMIL in HomeSite, SMIL export (for RealMedia) in Flash, and rumored upcoming SMIL support in DreamWeaver.
DoCoMo SMILEditor (Japanese site at http://www.docomo-sys.co.jp/prod/soft/smil2.html)
NCAM Media Access Generator "MAGpie" (http://ncam.wgbh.org/webaccess/magpie/) - A captioning tool for disability access.

Server-Side Generators: Companies developing server side solutions for SMIL tend to get gobbled up by larger companies wanting development platforms. But there are a few individuals and organizations developing server-side SMIL solutions. That is good because dynamic SMIL really shows off the power of an XML-based Web multimedia language:
Perly SMIL (http://www.webiphany.com/perlysmil/)
Smibase (http://www.smibase.com/)

SMIL Validator: Keep your code clean with a validator:
CWI SMIL Validator (http://www.cwi.nl/~media/symm/validator/)