In smplayer 0.6.2 there's a new dialog which allows you to find subtitle
files very easily.

The dialog can be opened with the option "Find subtitles on OpenSubtitles.org"
in the Subtitles menu.

It will automatically look for subtitles for the video you're currently
playing (or the previous video if none is playing).

The search is not done based on the file's name but on a hash which is
calculated from the file, so it doesn't matter if you renamed the file,
it will find the right subtitles for the file.

To download a subtitle, just double click on it or click on the Download
button. That would open your favorite web browser to actually download 
the file.

You can also upload subtitles to OpenSubtitles.org, you'll find an option
for that in the Subtitles menu. That would just open a web page with an upload
form (java required).