The ability to submit your tax return(s) online rather than printing and mailing.  This usually speeds up refunds, reduces or eliminates mathematical errors, while providing the IRS equivalent of proof of mailing.  Depending upon your income and the complexity of your tax situation, e-filing may be free, at least for the IRS federal tax return.  State tax returns are usually not free to e-file, despite their relative simplicity compared to the federal paperwork.