Over on his Facebook page, Fr. James Martin has some ideas:
The best gift of all is your love and prayers, but in case you want to supplement that, the most useful gifts (in my experience) are as follows, in order of cost. (By the way, none of the following companies put me up to this!)
First, the various “rites books,” which the priest can use for weddings, baptisms or funerals (normally a soon-to-be-ordained priest will already have the Roman Missal, a Lectionary, a breviary and so on, but still might need the rites books.) You can find them almost anywhere, but Catholic Book Publishing is a good start: http://www.catholicbookpublishing.com/list/category/liturgical+Editions
Second, a Mass kit, which a priest can bring with him to celebrate Mass where there might not be all the necessary supplies, etc.:http://www.catholicsupply.com/churchs/masskit.html
Finally, if you really want to give something memorable, a stole. The best color, to my mind, is gold and white, since you can use it almost anywhere (baptisms, weddings, etc.) The “gold standard” is made by the Trappists in Spencer, Mass., at the Holy Rood Guild. You can also get them to embroider a little greeting on the back. http://www.holyroodguild.com/xcart2/Canterbury_Stole.html
And the nicest ordination cards are from Printery House:
I can only add: Amen.
At my ordination several people gave me stoles, and my wife and her parents chipped in to buy me a couple beautiful dalmatics, too. Several people in the parish gave me gift certificates to a local church goods store. Another great resource, I think, is Deacon Store, which offers online gift certificates. My idol, Elizabeth Scalia, gave me a certificate from there when I was ordained. My goddaughter (whom I’d sponsored through RCIA, btw) gave me a nice chapel edition of the Sacramentary which I kept for reference at home (but with the new Missal, sadly, it’s out of date).
When in doubt, a little cash can’t hurt, along with any spare prayers!