UEFITool is a cross-platform C++/Qt program for parsing, extracting and modifying UEFI firmware images.
It supports parsing of full BIOS images starting with the flash descriptor or any binary files containing UEFI volumes.
Original development was started here at MDL forums as a cross-platform analog to PhoenixTool's structure mode with some additional features, but the program's engine was proven to be usefull for another projects like UEFIPatch, UBU and OZMTool.

You can either use pre-built binaries for Windows and OSX or build a binary yourself.
To build a binary you need a C++ compiler and an instance of Qt4/Qt5 library for it.
Install both of them, get the sources, generate makefiles using qmake (qmake UEFITool.pro) and use your make command on that generated files (i.e. nmake release, make release and so on).

The program can be started directly without any arguments or supplied with a single argument - a path to the UEFI image file to open after start.

The program window is divided into three panels: Structure, Information and Messages.
Structure of the image is represented as a tree of elements with different names, types and subtypes. If you select an element, Information panel will show the available information about the selected element based on it's type and contents.
Messages panel show all messages from the engine, including structure warnings and search results. Most of messages can be double-clicked to select the element that causes the message.

You can open a menu on each tree element to see what operations are possible for the selected element. This can include various types of Extract, Insert and Replace operations, as well as Remove and Rebuild.
Extract has two variants: Extract as is and Extract body. The difference is that Extract as is extracts the element with it's header (GUID, size, attributes and other structure-related information are located there), and Extract body extracts the element data only.
Replace has the same two variants as Extract with the same meaning.
Insert has the three different variants: Insert before, Insert after and Insert into, which is only available for UEFI volumes and encapsulation sections.
Remove marks an element for removal on image reconstuction.
Rebuild marks an element for rebuilding on image reconstruction. Normally, all elements that aren't marked for rebuild won't be changed at all and if you need to correct some structure error (i.e. invalid data checksum) you must mark an element for rebuild manually. If you change an element all it's parents up to the tree root will be marked for rebuild automatically. If UEFI volume is marked for rebuild all uncompressed PEI files in it will also be marked for rebuild because they must be rebased in the reconstructed image to maintain the executable-in-place constraint.

There is also a search function available from the File menu, you can search all tree elements for a specified hexadecimal pattern (spaces are not counted, dot symbol (.) is used as placeholder for a single hex digit), a specified GUID (rules are the same as for hex except for spaces) and a specified text (either Unicode or ASCII, case sensitive or not). Search results will be added into Messages panel, if anything is found.

After you've finished the modifications, you need to initiate image reconstruction using Save image file command from the File menu. If anything goes wrong on the reconstruction, an error will pop up, otherwise the program will prompt if you need to open the reconstructed file. Don't rush it, because reconstruction process can also generate some usefull messages, which will be lost if you open the reconstructed file immediatelly.