Which SQL Server edition and version you are using. In case you are using SQL Server 2008 Enterprise Edition you can enable backup compression which will give back some free disk space for your databases to grow further
Once you've established the free space and also feel confident that INSERTS won't occur soon that require the space, consider doing a on-off shrink. Before you try that step - consider doing some housekeeping on the server? Are there any other large files taking up space , on the same drives as where the data files are hosted? Can you move anything around? Even within the database itself , are there tmp tables which could be dropped etc. ?