If you're a home, office, or home-office user, then you'll probably be most concerned about how well your computer performs when completing general tasks and running productivity applications. Probably the easiest benchmark program for these general tasks is Novabench 2, a very basic program that performs RAM, CPU, HDD, and GPU tests. These are not complex nor intensive tests, but since you can compare your score online, you can easily get see if your PC is performing well compared to similar PCs, or if it has for some reason fallen behind.
Should Novabench's simple testing not satisfy you, there are some other programs that can help. For testing your CPU, CPU Free Benchmark 2.2 is a decent program. It simply monitors how long it takes your CPU to complete the testing phase - the lower, the better. What it lacks in information on the results screen it makes up for in intensity, as the benchmark will be using so much of your CPU's power that virtually no other program will operate at the same time (though you should turn off as much background stuff as you can to get an accurate benchmark anyways). For Hard Disks, there is simply no other tool that rivals I/O Meter, but be warned - this is a very technical program, so you'll need to read the documentation before you use it.
If you want a serious, all-in-one benchmark program, then SiSoftware's Sandra is probably a good bet. They offer a Lite version which disables many of the program's features, but still offers enough for home and office users. CPU, HDD, GPU, and multimedia benchmarks are all included. Just be wary, because the installation process of the version I installed offered a few extras that I didn't really want, and which some users may consider spyware. Make sure to decline installation of anything except Sanda Lite.