The 555 Timer chip

Terry Allen Steen, EE says

f = 1.44/[C*(Ra+2Rb)]
where

Ra is between Vcc (Pin #8) and Pin #7 (Discharge)
Rb is between Pin #7 (Discharge) and Pin #6 (Threshold)
C is between Pin #6 (Threshold) and Ground (Pin #1)

ensure Pin #4 (Reset) and #8 (Vcc) are tied together, and Pin 5 (Control) needs a filter cap if the circuit has noise problems.
ALWAYS A L W A Y S ALWAYS! Start with the cap. Set it by pulse width. Use this table:
Cap(uFd)    Pulse width(Seconds)
--------------------------------
10.0             90m -  1.2
 1.0              9m - 120m
 0.1            900u -  12m
 0.01            90u - 1.2m
 0.001            9u - 120u
--------------------------------
Select your cap, THEN select the resistors. If you want to make it easy, ASSuME the resistors are the same value, then you can use:
f = 0.48/R*C <==> R=0.48/f*C
If your resistor is abnormal in size, you have either chosen the wrong cap, or you are doing some kind of real odd freq.

Mark Willis says:

[The] original 555's which were sort of power hogs; Use a low-power CMOS version, and make sure you have that version's datasheet if you use one, if you go the 555 route. Also, [you] could drive a 555 at a higher frequency and divide it's frequency down by a proportional amount.
Electrolytics have huge temperature coefficients, using one for timing a 555, you want something like a Mylar or other low-variance cap. Those gre REALLY pricey IIRC by the time you reach 100uF, so use smaller C, larger R, and watch the construction, leakage on those pins 6 and 7 will change your timing.

Roman Black says:

I've been playing with 555s since 10 years old. You can also use a single inverter from a logic chip to replace a 555 in 95% of circuits. For a 14pin chip you can get 6 oscillators vs 2 in a 556.
{see: http://www.users.dircon.co.uk/~doctron/chuckles.htm look at the gated astable circuit}

Also:

The Dr. Calculus 555 astable calculator applet from pcbheaven.com