There are quite a few of these filters available, I think the Yahoogroups archive has one using blocklists, which works for 99% of all URL's (Including those without http:// , no www. , or IP addresses).
But since they break pages, I don't use them -- instead, I use an Opera feature: if you highlight text, you can send it to a search engine (defined in search.ini)
I added the following two 'search engines':
[Search Engine 12]
Name=Open URL (no &http)
URL=http://%s
Query=
Key=h
Is post=0
Has endseparator=0
Encoding=utf-8
Search Type=13
[Search Engine 13]
Name=Open U&RL
URL=%s
Query=
Key=r
Is post=0
Has endseparator=0
Encoding=utf-8
Search Type=13
Edit number and Key= for your search.ini
Using this tweak, I can open anything that looks like a URL with one mouse-click combo