Any TV or TV frequency monitor that has analogue RGB inputs (such as SCART supporting RGB) should be able to display the Atari low and medium resolution modes.
If you have a VGA monitor that can display TV frequency signals (horizontal frequency of 15625Hz and vertical frequency of 50Hz) then it may be able to display the Atari low and medium resolution modes. If the only input is the 15 pin VGA style D connector, you will have to make or find a suitable cable.
Some, but not all VGA monitors (CRT and LCD) can display the Atari high resolution mode. Results vary between different monitors, and the same monitor, but different Atari STFM/STE machines! You may well have to play with the monitors settings to get a full, and clear image. As the video output from the Atari does not completely match the VGA standards. Again, you will have to make or find a suitable cable or adapter.
Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (nearly forgot the Psion's)