via Bruna Lab with some additions by @Theunk
Or check my pinterestpage for a visual overview.

• ObsMapp from Observado / Waarneming over 10.000 additions each day
• SciSpy
• Project Noah
• Boskoi FoAM's Android app for mapping wildfood sources
• Inaturalist
• iMapInvasives all-taxa invasive species mapping tool; workes via phone's web browser
• PlantTracker invasives mapping University of Bristol
• CyberTracker
• WildObs
• iBats geotagged bat recordings

database apps:

• EpiCollect
• EpiSurveyor
• Pendragon Forms (Windows Mobile only)
• Open Data Kit includes built in publish data function to send completed data to an online database and from there, link in with Google's Fusion Tables and Earth.
• Fulcrum

• Movebank not specifically for mobile devices

• SoilWeb portable version of online interface to USDA-NRCS digital soil survey
• LightTracker Twilight, Sunrise, Sunset and solar noon information for any date, moon phases and rise for any date
• RiverFlows and WaterTracker view data from USGS and NOAA gauge sites to get real-time river-level information.
• Marine Weather Plus: marine forecasts, live buoy data, tides, radar, & sea surface temperatures

• iBird
• iTrack Wildlife
• Leafsnap
• Heukels high quality Dutch flora determination app €9,99
• Audubon field guides
• National Goegraphic Bird guide
• Sibley eGuide to Birds
• Cornell Lab of Ornithology Birdseye
• Guide to Trees of U Florida Campus
• Guide to Trees of OSU Campus
• NY Flora Association Blog: A list of field guides for iPhone/iPad has been put together by the NY Flora Association and can be found here. It includes guides to everything from plants to mushrooms.

• GPS Essentials
• iGIS
• MotionX GPS
• ArcGIS
• Google maps/Google Earth
• gvSIG mini
• GPS Status
• Nokia Maps: independent of a data signal (maps preload) and provides voice navigation, tilt and night views, 3D cities, and numerous gps-enhanced services as well as syncing with their full program on a PC
• EveryTrail: record trip route, add photos and videos, share route, and download to google earth
• PhenoMap: map plantphenology; upload images to a Flickr group.
• WolfGIS
• GISRoam
• QGIS for Android (Uunder development)

• Random Numbers
• Smart Measure: rangefinder; measure the distance to and height of an object
• Theodolite: overlays real time information about position, altitude, bearing, range, and inclination on the live camera image
• EMD Periodic Table of Elements
• TASA Geotimescale: timeline of eons
• Molecules: 3d models of molecules and enzymes

• Project Budburst: national field campaign designed to engage the public in the collection of data on timing of leafing, flowering, and fruiting of plants
• What’s Invasive!: submit geotagged photos and observations of invasive species
• IveGot1: Identify and report Invasive Species in Florida
• Nature's Notebook Android App: from USA-NPN's. Record observations on hundreds of species of plants and animals, as well as add new locations and species to your list in the field.
• iMapInvasives:online tool for invasive species reporting and data management.
• the USDA National Agricultural Library has a webpage with smartphone applications for invasive species monitoring and identification: link

• Skeptical Science
• GeographCA
• R Instructor: an interactive and multimedia tool designed to facilitate the learning process of R.