There are a good number of free worksheet sites online organized by topic and grade level, and some have worksheet generators where you can make your own custom sheets.
Kidszone - free worksheets by topic and by grade. Here is an example of their printable tracing worksheets.
Abc Teach: over 12,000 worksheets organized by grade and subject. Free tracing pages, including beginning penmanship. Here are some cute preschool/kindergarten level letter and shape tracing sheets.
Printfree.com: school printables like a teaching clock, custom graph paper, lined paper, and more.
A nice cumulative list of sources for free printables. I've used a few of these, DLTK and Enchanted Learning were my favorite when ds was younger. I'll be using them again when dd gets a little older.
Charlotte Mason manuscript copy work pages.
Worksheet generators, custom worksheets:
Personal Education Press: cards & signs, game boards, study sheets, quizzes, and flash games.
Penmanship worksheet generator(K-First)
Handwriting Worksheet Generator
Handwriting for Kids, includes left handed sheets
Free penmanship paper generator. Customize line spaces, color, paper size and line weight, very neat!
Math Fact Cafe(grade K-5), very nice worksheet generator and some premade worksheets.
Make your own penmanship worksheets by downloading fonts with and without dotted lines from Blue Vinyl. I use the Learning Curve and Print Clearly fonts. You can use these fonts in a word processor which allows you to use downloaded fonts, like your notepad or Microsoft Works. Unfortunately, Google Documents and free Jarte allow only their fonts. I've had limited success copying and pasting into these programs...still fiddling with it. I don't have Microsoft Office, and I'm still trying to download free Open Office(large download for dial up!) to see if their program is more flexible.
Discovery School's Worksheet Generator: These are great if you want to make your own multiple choice, matching, scrambles or word blank quizzes or worksheets. These can be used for anything - after a field trip, book reviews, vocabulary building, etc. There are also sheets for math, and some premade sheets. You can view samples before generating sheets.