My favorite piece of Mac OS software is Library Books. JD introduced me to this marvelous piece of software maybe about a year ago. It has saved me time and many library fines in the last year. Let me explain how & why.
Once installed, library books appears in the upper right toolbar by the clock and battery as a little star with a number by it. (Also called menu extras to a Mac expert. :)) By clicking on the star, a pull down menu appears. If LB has your library system in it, you can enter your library card number and it will download your record for you. Amazingly, LB has numerous library systems in it for most metropolitan areas. It can do more than one library card at a time (you will see JD and I both have a record). LB will tell you what you have checked out from the library in chronological order that they are due and will also tell you when your holds are ready for you. It also allows you to click on "renew items" and automatically opens the library's renewal page for you.
The star in the menu is the real gem of the software--it changes colors when different actions are needed. Black--all is ok. Red--the day before a material is due. Green--a hold is ready. LB is a great time saver--I can just click on it easily to see what I have checked out and when it is due.
The only downside I have found and haven't figured out a solution for is that I have my library card registered at more than one local library (different county system). Because it is the same card number, it can't seem to load both of my records at the same time. It just errors out, but I'm still playing with this problem and trying to figure it all out.