The NCEAS 'Forecasting Phenology' working group that Lizzie and Ben Cook led created the STONE (Synthesis of Timings Observed in iNcrease Experiments) and NECTAR (Network of Ecological and Climatological Timings Across Systems) databases.
Data and metadata from Lizzie's PhD research on the effects of non-native grass litter in coastal sage scrub communities are freely-available though the Knowledge Network for Biodiversity (KNB). GPS locations for plots are available upon request with approval from the field site's biologist.
Data for Lizzie's phenology review paper with Steph Pau are freely-available through KNB.
Fitter & Fitter's dataset of first flowering dates in Chinnor, England is available with their Science article.
The amazing long-term record at Gothic, Colorado, collected by David Inouye, is now hosted on the Open Science Framework (and amazingly recently recompiled by Jane Ogilvie, Brian Inouye, Nora Underwood with coding help from Leithen M’Gonigle). If you're interested contact them for how to request the data (or fill out this form).
NOAA maintains a short list of several phenology datasets here.
Much of Europe's phenology data (especially the International Phenology Gardens) were organized through their COST 725 action, which morphed into PEP725. See also PPODB, a database focused on Central Europe and its related article here.
The KNMI Climate Explorer site maintains a great set of organized resources.
And while we're on the topic of climate, check out the US Climate Resilience toolkit.
USDA PLANTS is a stand-by for US work, the BIEN and TRY initiatives have created large databases of plant functional traits, and hopefully will someday make their data fully publicly available. Also based in Europe is LEDA. For the British Isles Ecoflora is fantastic. Also, the Angiosperm Phylogeny Website has useful info, as does Watson and Dallwitz's Families of Flowering Plants.
Want to measure your own traits? Check out the trait handbook.
If you live in the northeast, check out GoBotany, which has an amazing online key. Also, I keep trying to remember MADHorse (maple, ash, dogwood, horsechestnut all have opposite branching).
I identified ants to species with help from the tremendous resources available at Antweb.
Harvard's MCZ museum has a good database with layered images of Caribbean insects.
Store your data (and share it) ... get started with these 10 Simple Rules.