Inexpensive Frequency Counter and Display for clock generator
A small micro would do this easily. You can set up a capture compare peripheral to count input pulses and read/clear at a regular interval (e.g. 50ms - use a timer interrupt for this)
Then write result (after multiplying by necessary value - e.g. in above example: capture_result * 20 = Hz) to whatever display you have in mind.  

A non-micro solution could be produced with some discrete logic ICs, there are quite a few designs floating around on the web. It would be a pretty "old school" solution but may be fine for a one off home project (and cheap) Here is a 0-40MHz example (from this page)



For higher frequencies a small CPLD/FPGA would work well and would be a quick/easy HDL program to write - may be an idea if you have a small dev board lying around.   

There are almost certainly dedicated ICs/Modules available if you have a look around. Most DMMs probably use an ASIC of some sort to provide multiple functions, though some of the cheaper models may use off the shelf stuff.