I personally like Jack Hale's book titled "Ordinary Differential Equations". I am a 3rd year graduate student studying Hamiltonian systems and have needed to review/learn topics in ODE's from time to time, and Hale's book has stood out as the most valuable resource for me.
I found Hale's book to be most readable, well-organized, and informative book covering the basics of ODE's. The treatment of the most basic issue of ODE's, i.e. existence/uniquness, is extremely well-written.
A lot of people seem to like Arnold's ODE book, and although it is a good book, I've had much better luck learning from Hale's book. Except for introducing differential equations on manifolds, all the main topics in Arnold's book are a subset of those in Hale's book. Hale also covers topics such as the Poincare-Bendixson Theorem and gets into stable/unstable manifolds, neither of which are present in Arnold's book. The presentation on time-periodic systems and related stability issues is also much clearer in Hale's book.
Another book I occasionally look at is Smale and Hirsh's book. This book is much more elementary than Hales and Arnold's, but has a few nice examples, especially in the few chapters regarding applications. However, the early section on The Poincare map is horrible, and should be the last place a person goes to learn what the Poincare map is.
To recap, if I need to look something up I looked at:
Hale
Arnold
Smale and Hirsch
Every time Hale's book wins in terms of readability, depth, and being easy to navigate through.
Last, but not least: Hale's book is published by Dover, and is quite cheap.