Zamzar is a free new services that allows you to convert just about any type of file into a new format without having to download special conversion software on to your computer. It works at converting image graphics files, music/audio files, video files, as well as Microsoft Office files. You just upload the file to their service, select the new format that you want the file in. They then perform the conversion and email you the resulting file.

Here's the list of file formats that they can convert to and from:

-- Music formats --
aac
ac3
flac
m4a
mmf
mp3
ogg
ra
ram
wav
wma

-- Video formats --
3gp
avi
flv
gvi
iphone
ipod
m4v
mov
mp4
mpg
ogg
rm
rmvb
vob
wmv

-- Image formats --
bmp
gif
ico
jpg
pcx
png
tga
tiff
wbmp
wmf

-- Doc formats --
csv
doc
docx
html
odp
ods
odt
pcx
pdf
ppt
ps
rtf
txt
wps
wpd
xls
xml