Opera is a very secure browser, and by default will only accept http, e-mail & ftp links. But you can tell it to accept Sig2Dat links too. :-)

1. Make sure Opera is off.
2. Open Program Files > Opera > Opera6.ini in a basic text program like Notepad. (The Opera6.ini file stores your preferences and set up of Opera.)
3. In section headed [User prefs], copy & paste this line of text:

TrustedExternalURLProtocols=sig2dat

If you'd like to use links for other file-sharing networks too, then use this line:

TrustedExternalURLProtocols=sig2dat, ed2k, magnet

4. Save the Opera6.ini file, and close it. Opera will now recognise links starting with sig2dat as allowed links.

*** Opera 7.20 and upwards also supports a new method of allowing Sig2Dat links, by adjusting the program preferences.***
See Opera Knowledge Base - Handling Unknown Address Types.

One issue with Opera is that it renders a link's URL according to the standards set out for regular web links, i.e. no spaces & all lower-case letters. FastTrack needs the hash letters in their original state, both upper & lower case, and Sig2Dat needs spaces to separate different sections of the URL. So the URLs of links made in earlier versions will look like this when you hover your mouse-pointer over the link:

sig2dat://|file:%20virtualdub-1_4_10.zip|length:%20711077
%20bytes,%20694kb|uuhash:%20=6qbuemkairmrm7ubmzt9sqeu6gq=|

Notice how everything's in lower-case and "%20" instead of spaces? If you click on it, yes a Start file will be created, the download will appear in your Traffic window, but it will never actually download.
Indy eliminated the %20 issue in an earlier version. Now with 3.11.a, he worked out how to keep letters in their original case... just by adding an extra slash after sig2dat:// ! :-)

sig2dat:///|File:VirtualDub-1_4_10.zip|Length:711077
Bytes,694KB|UUHash:=6QbuEMkaIrmrm7UBmZT9SqEU6GQ=|

For some reason, the three-slash link works for Opera, and can still be used by Internet Explorer and Mozilla.