You have just downloaded the latest version of Unreal Engine 4. Congratulations ! But you might need a few additional tools so here's a list of stuff I like to use. They're all free because times are tough.
3D models : Blender
UE 4 only accepts .obj and .fbx when it comes to importing 3D models. Blender can be useful if you get .3ds files (or other formats) and you need to convert them to .fbx. Also you might need to tweak the 3D models in Blender before exporting like modifying the scale, changing the orientation...
2D graphics : Paint.net
It's not as powerful as Photoshop but it's also not as messy and it runs way faster. Whenever you need to modify textures/photos/UI elements, it gets the job done. It supports layers and offers all he basic tools you need.
C++ : Visual Studio Express 2013 For Windows Desktop
Blueprints are cool but sometimes you have to code some C++. If you work on Windows, you need Visual Studio since UE generates Visual Studio solutions. It's also a very powerful IDE and the free version has everything you need to compile and debug.
Text editor : Notepad++
A ini file to tweak ? A cpp file to modify without opening the entire solution ? Notepad++ is here for you.
Audio : Audacity
Audacity allows you to easily edit audio tracks, mix them, add effects and output the result. Use Wave files as your output format. UE4 does not accept MP3/OGG and will do its own compression at import.
Video : Freemake Video Converter
The best and the simpliest media converter. Just select your audio/video files, choose the output format (MP4/WMV for UE) and run. Be careful during the install : the installer will try to install a lot of crapware so choose the custom install and say no to pretty much everything.
Documents/Spreadsheets : Google Doc/Google Sheets on Google Drive
First of all, you should never have any import document offline. Everything should be in the cloud. Google Drive is by far the best solution for that. Not only it allows you to add and modify documents online but you can also access them on any device/computer and share them with other people. And multiple people can work at the same time on the same document without overwriting someone else' changes. I use Google Docs for simple notes and game design documents and I use Google Sheets to track bugs and features to implement.
Google Drive is also a nice place to put your latest public build.
Assets/Code backup : Box.com
All your code and all your assets have to be saved in the cloud. Not on an external hard drive. Not on a personal home server. Those things can fail. Your cat can pee on it. Your jealous ex-girlfriend/boyfriend can set it on fire. When your assets are in the cloud, they're handled by professionals which reputation depends on not loosing your data. I prefer Box.com over GitHub and Dropbox because the free version offers 10 GB of storage and allows to upload large files (250MB) which is good because UE maps can get really big.
For $10 a month, you get 100 GB, the file size limit is 5 GB, you can share your data with 10 users and you have a versionning system that allows to go back to 25 previous versions. Here's what you should upload for each UE project :
-Binaries : yes but remove the .pdb files first
-Build : yes
-Config : yes
-Content : yes
-Intermediate : yes but remove the .pdb files first
-Saved : no except for the sub-folder Config
-Source : yes but remove the .pdb files first
Project.sln : yes
Project.uproject : yes