I was working on a 2D game in Unity and needed some way to automatically create a collider for a complex 2D level mesh. Editing a Polygon Collider 2D in the editor was out of the question, since there were thousands of vertices. I ended up writing a script that analyzes the MeshFilter of the mesh, and programmatically creates a Polygon Collider 2D for it. It also supports meshes that have mutliple non-connected components.

How to Use It
To use it, create a ColliderCreator C# script, copy the code in, and add the Script component to a GameObject that has a 2D mesh. When you hit Play, the script will generate a Polygon Collider 2D for that GameObject based on the mesh. While the game is running, it's best to save the generated PolygonCollider2D using Copy Component, and paste it into the prefab. Then the script won't have to run every time.

ColliderCreator C# Code
using System.Collections.Generic;
using UnityEngine;

public class ColliderCreator : MonoBehaviour
{
void Start()
{
// Stop if no mesh filter exists or there's already a collider
if (GetComponent<PolygonCollider2D>() || GetComponent<MeshFilter>() == null) {
return;
}

// Get triangles and vertices from mesh
int[] triangles = GetComponent<MeshFilter>().mesh.triangles;
Vector3[] vertices = GetComponent<MeshFilter>().mesh.vertices;

// Get just the outer edges from the mesh's triangles (ignore or remove any shared edges)
Dictionary<string, KeyValuePair<int, int>> edges = new Dictionary<string, KeyValuePair<int, int>>();
for (int i = 0; i < triangles.Length; i += 3) {
for (int e = 0; e < 3; e++) {
int vert1 = triangles[i + e];
int vert2 = triangles[i + e + 1 > i + 2 ? i : i + e + 1];
string edge = Mathf.Min(vert1, vert2) + ":" + Mathf.Max(vert1, vert2);
if (edges.ContainsKey(edge)) {
edges.Remove(edge);
} else {
edges.Add(edge, new KeyValuePair<int, int>(vert1, vert2));
}
}
}

// Create edge lookup (Key is first vertex, Value is second vertex, of each edge)
Dictionary<int, int> lookup = new Dictionary<int, int>();
foreach (KeyValuePair<int, int> edge in edges.Values) {
if (lookup.ContainsKey(edge.Key) == false) {
lookup.Add(edge.Key, edge.Value);
}
}

// Create empty polygon collider
PolygonCollider2D polygonCollider = gameObject.AddComponent<PolygonCollider2D>();
polygonCollider.pathCount = 0;

// Loop through edge vertices in order
int startVert = 0;
int nextVert = startVert;
int highestVert = startVert;
List<Vector2> colliderPath = new List<Vector2>();
while (true) {

// Add vertex to collider path
colliderPath.Add(vertices[nextVert]);

// Get next vertex
nextVert = lookup[nextVert];

// Store highest vertex (to know what shape to move to next)
if (nextVert > highestVert) {
highestVert = nextVert;
}

// Shape complete
if (nextVert == startVert) {

// Add path to polygon collider
polygonCollider.pathCount++;
polygonCollider.SetPath(polygonCollider.pathCount - 1, colliderPath.ToArray());
colliderPath.Clear();

// Go to next shape if one exists
if (lookup.ContainsKey(highestVert + 1)) {

// Set starting and next vertices
startVert = highestVert + 1;
nextVert = startVert;

// Continue to next loop
continue;
}

// No more verts
break;
}
}
}
}

Old Version

The old version of the code was much slower. You can still see it below, but I highly recommend using the above one.

using UnityEngine;
using System.Collections.Generic;

public class ColliderCreator : MonoBehaviour
{
private int currentPathIndex = 0;
private PolygonCollider2D polygonCollider;
private List<Edge> edges = new List<Edge>();
private List<Vector2> points = new List<Vector2>();
private Vector3[] vertices;

void Start()
{
// Get the polygon collider (create one if necessary)
polygonCollider = GetComponent<PolygonCollider2D>();
if (polygonCollider == null) {
polygonCollider = gameObject.AddComponent<PolygonCollider2D>();
}

// Get the mesh's vertices for use later
vertices = GetComponent<MeshFilter>().mesh.vertices;

// Get all edges from triangles
int[] triangles = GetComponent<MeshFilter>().mesh.triangles;
for (int i = 0; i < triangles.Length; i += 3) {
edges.Add(new Edge(triangles[i], triangles[i + 1]));
edges.Add(new Edge(triangles[i + 1], triangles[i + 2]));
edges.Add(new Edge(triangles[i + 2], triangles[i]));
}

// Find duplicate edges
List<Edge> edgesToRemove = new List<Edge>();
foreach (Edge edge1 in edges) {
foreach (Edge edge2 in edges) {
if (edge1 != edge2) {
if (edge1.vert1 == edge2.vert1 && edge1.vert2 == edge2.vert2 || edge1.vert1 == edge2.vert2 && edge1.vert2 == edge2.vert1) {
edgesToRemove.Add(edge1);
}
}
}
}

// Remove duplicate edges (leaving only perimeter edges)
foreach (Edge edge in edgesToRemove) {
edges.Remove(edge);
}

// Start edge trace
edgeTrace(edges[0]);
}

void edgeTrace(Edge edge)
{
// Add this edge's vert1 coords to the point list
points.Add(vertices[edge.vert1]);

// Store this edge's vert2
int vert2 = edge.vert2;

// Remove this edge
edges.Remove(edge);

// Find next edge that contains vert2
foreach (Edge nextEdge in edges) {
if (nextEdge.vert1 == vert2) {
edgeTrace(nextEdge);
return;
}
}

// No next edge found, create a path based on these points
polygonCollider.pathCount = currentPathIndex + 1;
polygonCollider.SetPath(currentPathIndex, points.ToArray());

// Empty path
points.Clear();

// Increment path index
currentPathIndex ++;

// Start next edge trace if there are edges left
if (edges.Count > 0) {
edgeTrace(edges[0]);
}
}
}

class Edge
{
public int vert1;
public int vert2;

public Edge(int Vert1, int Vert2)
{
vert1 = Vert1;
vert2 = Vert2;
}
}