digital trim pot?

has anyone seen such a device/ and used them? say with a ne555?

I've used them, they're very handy for trimming a bunch of analog values, and many of them save the settings in nonvolatile memory so it works on power-up. They don't have real fine adjustments, most aren't more than 256 levels, but with care you can get a lot of mileage out of that. They generally need a serial port - SPI or IIC - to operate them, but there are a few with "up/down" controls. Check Digikey and Mouser.

I have used the DS1869, which allows separate up/down buttons and can operate free of an MCU.

Since you mention using a 555, it may be worth noting that digital pots are typically low current and low voltage devices. The DS1869 is limited to 5 V across its terminals (excluding stress ratings).

John