SMIL 2.0:SMIL 2.0 implementations are still in their early stages but you will find a range of powerful solutions from large and small software vendors alike:
RealNetworks RealOne (http://www.realnetworks.com/solutions/ecosystem/realone.html) - A full Web multimedia browser and player with significant SMIL 2.0 support.
Microsoft Internet Explorer (http://www.microsoft.com/windows/ie/default.asp) - SMIL 2.0 support mixed in with HTML in their Windows version of the browser.
ORATRiX Development GRiNS-SMIL 2 Player (http://www.oratrix.com/GRiNS/SMIL-2.0/) - Cross-platform SMIL 2.0 support from a small company that still shows the big companies how to get the job done.

SMIL 1.0: SMIL 1.0 players do not pack the punch of their younger SMIL 2.0 siblings but you will find wider adoption and a longer track record for more authoring stability:
Apple QuickTime 5 (http://www.apple.com/quicktime/download/) - Simple cross-platform SMIL support initially and primarily intended for sequential stacking of content.
RealNetworks RealPlayer 8 (http://www.real.com/) - Solid cross-platform SMIL 1.0 support with proprietary RealPix and RealText implementation for more power with images and text. RealPlayer implementations will gradually fade as RealOne comes into dominance.
Helio SOJA "Cherbourg" (http://www.helio.org/products/smil/) - The most refined Java based SMIL player.
X-SMILES (http://www.xsmiles.org/) A Java based XML/SMIL browser.
SunTREC Schmunzel (http://www.salzburgresearch.at/suntrec/schmunzel/) - Another Java based SMIL player.
NIST SMIL Player (http://smil.nist.gov/player/) - Yet another Java based SMIL player.
Compaq HPAS (http://www.research.compaq.com/SRC/HPAS/) - Rare Java and stand-alone implementation.