Two online verb conjugators that I regularly use are at verbix.com. 

One is for (modern) Icelandic, and the other is for Old Norse. 

In theory the Old Norse verb conjugator is all I would need, but it's newer and not as complete. 

In any case, both are useful if you are not sure whether for example, "sé" is a form of "vera" or "sjá." (Turns out that it could be either, but first person present subjunctive for "vera," and first person present for "sjá.")