NMR solvent peaks
I just noticed this mentioned on Twitter and so I've added it to the Mobile Science site.
NMR Solvent peaks is a conveniently-searchable version of the ungainly table of NMR data most organic chemists keep a copy of nearby. Instead of searching through the table for a peak near your unidentified peak, just enter your solvent and the peak's multiplicity and location and you'll have a short list of candidate impurities
There are a number of other NMR apps available