After almost 4 years in the hobby I've acquired a bunch of stuff that gets used quite frequently, some more than others. These are items frequently found lying around the house, most of which are multi purpose and get used for other things.
Here's 20 items that beginners may find useful at some point during their time in the hobby. These aren't all necessary tools for beginners but they may help at some point making your life easier
Top 20 tools
1. Syringes - Inject pest anemones, feed LPS corals, dose etc...
2. Bottle brush - Clean plumbing and other items
3. Neodymium magnets - mount things such as frag racks to glass or back wall
4. Flashlight - inspect tank when lights are out for creatures/pests
5. Binder clips - clip stuff to edge of tank for acclimating new additions
6. Bone cutters - cut/trim SPS corals and remove frags from plugs etc...
7. Single edge razor blades - clean glass, frag soft coral. Consider these single use as they will rust quickly
8. Hydrogen peroxide 3% - dilute with tank water and dip some types of corals to kill most types of algae as well as pests such as pods, starfish, crabs and nudibranches
9. Bayer Insecticide - dip all new coral additions diluted with tank water to kill pests such as pods, starfish, crabs, nudibranches
10. Muriatic Acid - dilute with freshwater to clean pumps, powerheads and other submerged equipment
11. Cyanoacrylate kicker - speed up cure time of crazy glue
12. Strainer - strain frozen foods before feeding fish and corals
13. Infrared thermometer - check tank temps to ensure accuracy, compare tank and new salt water temps
14. Fricken Laser - burn stuff such as pests, Coraline algae or cheaper corals from encroaching on nicer corals
15. Fruit bags - hold hydroton for greater biological filtration, carbon, and rinsing live sand depending on mesh size
16. Digital scale - weigh salt for making new batches of salt water and mixing your own 2 part recipe solutions etc...
17. Turkey baster - blow detritus off rocks helping prevent algae growth
18. Rubble tray - frag soft corals such as mushrooms and zoanthids creating a low flow area and rubble to attach to
19. Egg crate - build racks for growing out frags, keep frags separate
20. Carbon fibre rods - move things around such as frags, flip snails over etc...
Some of these items will require you to do some research, such as item #8, 9, 10, 11 and 14