Cyberhymnal.org is a wonderful resource. There are so many verses in so many old hymns that I've forgotten, and even more that I never even knew existed. If you use your computer to access it instead of mobile, there are even midi files for you to learn the classic hymntunes.
But its most valuable bit is that it's always with me because my phone is always with me. And so when Big Frog and I were listening to a hammer dulcimer hymns CD on a road trip, and I was chiming in on just the chorus, he called out, "Now for the mumbling part!" and I shamefacedly pulled out the Magic Hymnal.
(On the plus side, even though I couldn't come up with any of the verses of Tis So Sweet to Trust in Jesus, out of my head, when I saw them, I did know all 4 verses. Also 7 verses of Amazing Grace, although I would put them in a different order than cyberhymnal had.)