Sometimes you just want to know that the disk you're going to use is in good condition and your disk drive gets along nicely with it.  Here's a simple but effective tool for testing that disk.  1541 Sector Tester will read every track and sector on the disk and let you know if an error is returned.

Disc Rescue v1.0 by Cal/Alien - Released in 1990.  This utility has three main functions;  unscratch files, un-new a disk and remove errors.  Although the program name is spelled dis(c) and not disk, I can assure you it isn't for a CD. ;)

Discan v1.0 by MR. NOP 

This program checks an entire disk for a series of bytes, and allows you to change those bytes if found. This allows you to create game cheats, remove copy protection and to do virus scans. What makes this program useful is that it allows you to modify data in games that don't contain files. With files, it's easy to modify code and create trainer menus. When the data is hidden on sectors though, it's not as simple a process because you need to modify and then re-save the data back to disk. 

See detailed description for more instructions.

Disk Check-up by Don Fulton is a nice disk checker for the Commodore 64 that will not only check the disk, but will trace down which files in particular are affected by a track/sector error.

Written by Jim Butterfield. Disk Checker has a few functions which can aid in identifying a bad disk. You can check all the files, check for bad spots or recover scratched files.

Disck Checker+ appears to be a modified version of Jim Butterfields Disk Checker.  This particular version seems to scan for errors 20, 21 and 23.

Disk Compare - compares the disk in drive #8 with the disk in drive#9 to see if they are identical.

Release on March 10th,1986 by the Rancocas Valley User Group. Disk compare isn't fancy but when you want to see the difference between two floppies and you have two 1541 drives, then this utility will help.

Since I can't read the text in the program I can only tell you that I think this program would be perfect for cleaning drive heads with one of those cleaning disks.  Not only does it spin the disk, but it moves the head from track 1 to 35 over and over.

Disk Fixer scans the disk for errors (bad blocks) and allocates the blocks so the dos will not save files to the bad blocks.

Written May 1986 by Bummy. Disk Scan is a simple program that will graphically display the used tracks and sectors on a disk with an optional hard copy at the end.

Disk Tester v1.0 by Mr. Fiz is a small but fast disk checker that is capable of testing for quite a few error types.  Check out the built-in help screen for more details.

Disk Timer - Checks the disk rotation speed by writing data onto the disk, then reading it back in.

Written by Kevin Pickell, Diskette Examine will check a disk for errors. There are two modes; A quick mode which only takes a handful of seconds and a full mode that thoroughly examines tracks and sectors.

Error Check 64 is a utility for the Commodore 64 to check a full disk for track / sector errors.

Floppy Speed Test is a disk drive speed tester from the magazine 64'er.  Although it's written in German, it is easy enough to understand the results.

Written by Will Corley, Katscan 1541 is a simple program that will scan a full disk and tell you the checksum value.  Great for making sure your duplicate is 100%.

Naughty Blox v1.1 by Will Corley is a utility for the Commodore 64 that will check the blocks (tracks/sectors) of a floppy disk to make sure it's all readable.

Note: There is a newer version of this on the site, but I like to keep around old versions for history's sake.

Written by Will Corley, Naughty Blox is a disk checker and checksum generator.  It will also validate a disk.

This super simple utility can check a disk for errors by adding up the blocks used by files and the free blocks and compare it to known total blocks for formatted floppies on 1541,1571,1581 drives.

This program fixes a 1581 floppy with a short-header (no floppy ID).

Superclone Disk Examiner is  a disk utility that will let you verify the checksum of tracks 1-40 and can accomodate certain tracks that might have extra sectors. 

Some additional information about checksums I found at the excellent pagetable.com website:

The on-disk format of 1541 disks uses 8-bit checksums to protect the integrity of headers and data sections. It is calculcated as an XOR of all data bytes. An 8 bit checksum is not very strong: The probability of an undetected read error is 1:256. In practice, it is good enough that it should be trusted for most cases.

Very small but handy little utility to let you test a disk track for errors on the Commodore 64.

Here's an anonymous utility that has a couple nice features, such as checking the disk for bad blocks, viewing the load address of a machine language program, change a disks name or id, and deleting files.

Virus Kill 2.4 programmed by Exen/Fatum.  Yup, even the Commodore 64 had viruses.